1. Field of the Invention
The present invention relates to a surface light source used in a liquid crystal display (LCD), and particularly to a surface light source and a light guide plate used therein having uniform luminance.
2. Prior Art
In general, LCDs have two main advantages in comparison with cathode ray tubes (CRTs): LCDs are thin, and have low power consumption. It has been said that LCDs might one day completely replace CRT display devices, and LCDs have aroused great interest in many industries in recent times. In general, an LCD needs a surface light source to provide even light for a clear display.
A surface light source comprises a light source and a light guide plate. The light source may be a linear light source, or one or more point light sources. The light guide plate has an end face through which light is introduced, and two opposite major faces one of which functions as an emission face. The performance of the surface light source greatly depends on the characteristics of the light guide plate employed therein.
A light guide plate functions to change a direction of propagation of light beams emitted from the light source and introduced into the light guide plate, from a direction roughly parallel to the emission face of the light guide plate to a direction perpendicular to the emission face. That is, the light guide plate effectively changes the linear or point light source(s) into a surface light source, for evenly illuminating a whole display screen of the LCD. The light guide plate comprises a plurality of scattering dots disposed on the bottom surface, for eliminating total internal reflection of light beams in the light guide plate. That is, the scattering dots diffuse the light beams, thereby improving the uniformity of brightness of the light guide plate and enhancing the optical characteristics of the LCD. Different LCDs require scattering dots having different shapes, sizes, and distribution densities depending on the different uses to which the LCDs are put.
FIG. 5 shows a conventional surface light source 10, which comprises a light guide plate 120 and a linear light source 110 adjacent to one side of the light guide plate 120. The light guide plate 120 generally defines two so-called dark areas 130 at two corners thereof adjacent to the light source 110. This is due to the linear light source 110 providing uneven illumination, the linear light source 110 being brightest along a main middle portion thereof.
FIG. 6 shows another conventional surface light source 20. The surface light source 20 comprises a light guide plate 220, three aligned light emitting diodes (LEDs) 210 adjacent one side of the light guide plate 220, and a reflector (not shown). Each LED 210 emits light beams over a limited predetermined range of angles, and the light beams enter the light guide plate 220 with uneven distribution. As a result, four dark areas 230 are defined in the light guide plate 20. The luminance of the four dark areas 230 is less than that of a remaining main area of the light guide plate 20. The surface light source 20 cannot attain uniformity of light beams exiting therefrom.
A new surface light source and a light guide plate for the surface light source which overcome the above-mentioned disadvantages are desired.